Paper machines or the like utilize press felts for absorbing water from a fiber web to increase its dry matter content. As used in this application and claims, the term paper machine refers to a paper machine, a board machine or other similar machine producing a web-like product from a fiber suspension. The absorbency of the press felt, typically described by its air permeability (m.sup.3 /m.sup.2 min), decreases when the felt is compressed during operation in a nip formed by rolls. The air permeability of a new felt is typically about 15 m.sup.3 /m.sup.2 min, and that of a felt which has totally lost its elasticity is about 2 to 1 m.sup.3 /m.sup.2 min. Particles detached from the paper web also block the felt, but not to an equally high degree as the compression effect.
In principle, the compression takes place evenly when the press effect and structure of the rolls are perfectly even and symmetrical. If the press effect created by the rolls is asymmetric or the rotation of the felt is realized by so-called cambered rolls, the felt is compressed unevenly and asymmetrically. In the latter case, the central portion of the felt stretches more than the rest of it, so that its thickness and thus its water volume is smaller as compared with the edge portions. Paper machines also comprise so-called wet wires, the purpose of which is to transport away water removed from the fiber suspension and from the forming or formed fiber web. Such wet wires are also exposed to compression and they are blocked by loose material gathering in them similarly as other felt types.
A partly or totally blocked felt can be reconditioned by using high-pressure needle-like water jets, which are in general use today. Such needle jets increase the porosity of the felt and retard its ageing and blocking. In general, such water jets are effected by means of jet pipes provided in the apparatus so as to oscillate in the transverse direction of the felt. The pipes comprise needle nozzles at predetermined intervals, and the oscillating movement of the pipe is approximately equal to the distance between the needle jets. Particles gathered in the felt are removed by applying an ordinary detergent to the felt during a break in operation and allowing it to influence for some time, whereafter it is rinsed off. Wet wires are cleaned by means of needle jets similarly as felts, and their condition has to be monitored in a similar way. The term felt will be used below in this application and claims to refer to a felt, a wet wire and any other fabric used for similar purposes in a paper machine.
Various arrangements have been developed for reconditioning a felt and measuring its condition. GB Patent 1 458 294 discloses an arrangement utilizing a suction shoe provided with an elongated slit. A needle-like water jet is applied from one end of the slit to the felt, and the water sprayed into the felt is sucked through the slit into a vacuum chamber provided above the slit and removed through a suction conduit. The chamber is connected to a vacuum conduit and it communicates with the atmosphere only through the slit. The suction shoe is mounted in a beam extending in the transverse direction of the felt, and it is movable along the beam across the felt in its transverse direction, so that the needle-like water jet can be applied over the entire width of the felt. This arrangement is specifically intended for reconditioning although it is stated in the publication that the device can be used for analyzing the condition of the felt when the water jet is switched off. In practice, the air permeability of the felt cannot be reliably analyzed by means of this arrangement during normal operation because water from the wet felt blocks rapidly the discharge conduit or forms plugs distorting the measuring results so that the measured values are not logically proportional to the condition of the felt. Furthermore, the use of this arrangement for analyzing the condition of the felt is impossible in practice as the comparison should be made by reading a vacuum gauge connected to the vacuum conduit. In practice, it is not possible to observe sufficiently accurately the vacuum and determine manually the position of the measuring head with respect to the felt.
U.S. Pat. No. 3,056,281 discloses an arrangement for measuring the condition of a felt. It comprises sensors provided with a chamber, a suction conduit leading out of the chamber and a pressure gauge connected to measure the pressure of the chamber. The measuring surface of the sensor is provided with holes leading out of the chamber. The condition of the felt is measured by means of the device by positioning the surface with the holes against the felt while the operator observes the pressure values of the chamber from the pressure gauge. Although the arrangement can be used for measuring the condition of the felt, it is impossible in practice to mark the measured values accurately with respect to the surface of the felt so that it is impossible to monitor the condition of the felt as a whole. A problem with the arrangement of this publication, too, is that water from the wet felt easily blocks the suction conduit, causing abrupt illogical variation in the measured values so that the obtained information on the condition of the felt is not reliable.
CA Patent 1,143,982 discloses an arrangement in which the condition of the felt is monitored by measuring vaccum pressure in a conventional suction box and by switching on the cleaning jets over the entire width of the felt when the pressure drops below a predetermined limit and correspondingly switching off the jets when the pressure in the suction box rises, indicating improved air permeability of the felt. In this arrangement, it is not possible to locate the blockage in the felt, so that the reconditioning cannot be directed merely to the required area.
U.S. Pat. No. 3,762,211 discloses a solution in which the porosity of the felt is measured in its transverse direction by means of suction boxes mounted over the entire width of the felt. The suction boxes are divided in the transverse direction of the felt into several successive chambers and the boxes suck air and water through the felt, so that the flow rate of air removed at each compartment is measured by means of a separate measuring head. In this solution, water and air are passed apart from each other outside the primary suction chamber in order that the air flow could be measured. Water and air are, however, removed from the suction box along one and the same conduit, so that the air flow and water amount of all compartments are removed simultaneously. A drawback of this solution is that the measuring result of the air flow of each compartment is dependent on the air flow of the other compartments, and that the mixing of water and air in the common discharge conduit causes random variation in the measuring result, so that reliable information on the condition of the felt at its each particular point cannot be obtained.